Rachni

The Rachni are a highly intelligent, space faring insect race that had been thought to be wiped out millenia before the setting of the original Mass Effect. The Salarians encountered the Rachni upon activating a newly discovered mass relay leading to Rachni controlled space. Unfortunately, the Rachni species had been subjugated by the reaper Sovereign through indoctrination, and proved extremely hostile. It is a likely hypothesis that Sovereign intended to use the Rachni to conquer the council races so that it would be able to regain control of the citadel. This plan would have likely succeeded if not for the Krogan. Their arrival onto the galactic scene turned the tide of the war in the citadel species' favor.

During the course of Mass Effect, players encounter the long extinct species on the planet Noveria, where an egg found in a derelict ship had been brought by one of the corporations based there. This corporation happened to have Saren as an investor, and he used his access to begin a project with two goals. The first, was to use the Rachni Queen hatched from the egg to breed an army of rachni soldiers. The second was to extract the location of a long lost Mass Relay from the ancestral memory of the Rachni Queen utilizing the unique mental abilities of the Asari.

The player has the option of killing the Rachni queen in Mass Effect or it can be set free. If it is set free an Asari messager on Illium tells Shephard that they are planning to help him "against the darkness".

The Rachni Queen is the matriach of the species and raises her children as she sees fit. The Queen in Mass Effect states that if her children are not raised by her they will be wild and uncontrollable. She explains this and gives Sheppard permission to kill her children on Noveria before the option of how to deal with her becomes available. There are three other types of Rachni.

Rachni Workers are small, green and attack in swarms. They are the first line of defense for the Rachni. They rush towards their enemy and explode causing toxic damage.

Rachni Soldiers are much larger and have several tentacles attached to their body. They use these to attack their enemies along with a toxic spit.

Rachni Brood Warriors are the elder males of Rachni hives. They are much larger than Rachni Soldiers and seem to rival the Rachni Queen in size. They typically stay underground until the hive is strained for warriors or in peril. They are formidable opponents since they contain stronger versions of the Rachni Soldiers abilities and also have biotics. These abilities include warp, stasis, and some form of barrier.

Ravagers are the cybernetically modified version of Rachni that are slaves to the Reapers. Although the bulk of the Rachni are under the control of the Rachni Queen, the Reapers have pockets of Rachni Ravagers under their controls